The obvious touchstone for Geoff Dyer's tenth book, which happens to be on photography, is his previous book on jazz, But Beautiful, which was written from the point of view of someone who simply liked listening to jazz, but nevertheless managed to be both erudite and illuminating. This offering, which is ambitious in its scale - 42 photographers are discussed, from the early 1800s to the present day - roams freely across decades, forms and subjects without losing its unifying thrust. Curator John Szarkowski once said of Garry Winogrand's photographs that they 'were not illustrations of what we had known, but were new knowledge', and it is this 'new knowledge' that Dyer sets out to decode in his inimitable way.

He begins, as is his wont, by alerting us to his apparent unsuitability for such a task: 'I am not a photographer ... I don't even own a camera.' This admission made my heart sink for a moment. There was a time in Dyer's writing life when the fact that he did not own a camera would have spawned an entire chapter in itself, maybe even a book. Thankfully, those days have gone. Here, he dives sideways into his exploration of photography by citing Borges, a blind writer, who once wrote of a 'certain Chinese encyclopaedia' which categorised animals in a rigorous, but eccentric, manner: 'stray dogs', 'mermaids', 'those that tremble as if they were mad' and so forth. While Dyer's book is neither as eccentric nor as rigorous, it does set about the task of elucidating what Henri Cartier-Bresson famously called the 'decisive moment' in a singular and illuminating way.

Rather than weigh up the work of selected photographers from the last century as an academic might do, Dyer has opted, instead, to look at certain subjects common to all the photographers he admires: benches, roads, doors, blind people, hats, fences, streets. His task is to try to define the differing styles and sensibilities that make those subjects appear both definably similar and infinitely different.

From Borges, whose most arresting portrait, incidentally, was taken in Central Park by Diane Arbus, Dyer moves on to images of anonymous blind people, who were photographed so frequently in the early half of the last century that Evans wrote about encountering 'the inevitable familiar inventive blind man'. Paul Strand, Lewis Hind, Garry Winogrand, Ben Shahn, Andre Kertesz, as well as Walker Evans and Arbus, all made dramatic images of blind people on the streets of America across the last century.

Many of their subjects were itinerant musicians; others begged with signs around their necks advertising their affliction. Some were startled by the sudden click of a shutter; others went on their way oblivious to their fate, which was to end up on gallery walls, forever stared at, forever unknowing.

As a symbol of photography's power to make us look at what we would otherwise overlook, and its ability to disempower its subject, 'the inevitable familiar inventive blind man' takes some beating.

The blind beggar turns up again during Dyer's equally illuminating discourse on hats in photography. He's there, standing outside a bank, cap in hand, in John Vachon's straightforwardly titled Blind Beggar, Washington DC, November 1937. As Dyer points out, the hat, whether pristine or crumpled, ostentatious or sweat-stained, is one of the great signifiers of both great wealth and extreme poverty in Thirties America.

There is no more affecting image of the dignity of working men brought low by economic circumstance than Dorothea Lange's White Angel Bread Line, San Francisco, 1933, in which the hats, some battered, some maintained against all the odds, seem to say as much as the various turned backs and the few faces that gaze expressionless at the camera.

Dyer seasons all this cultural exploration with anecdote and opinion, and, unsurprisingly, has his often eccentric take on the photographs he has chosen. It is almost worth the price of admission alone for his frankly sexual, and often hilariously funny, account of the tangled relationship between photographers Edward Weston, Alfred Steiglitz and Paul Strand, and the object of their photographic and voyeuristic affection, artist Georgia O'Keefe.

He's good, too, on the mystery that is William Eggleston, whose pioneering work in colour, though approaching the garishly vulgar, remains as elusive as it is influential.

'I suspect that this book will be a source of irritation to many people,' Dyer says in his introduction, 'especially those who know more about photography than I do.' Perhaps. The thing is, though, Dyer knows quite a lot about photography, and his odd and illuminating book may yet takes its place among the classic works on the medium.